The sporulating, obligate anaerobic, grain-positive bacillus Clostridium produces eight forms of antigenically distinct exotoxins. Tetanus neurotoxin (TeNT) is produced by Clostridium tetani while Clostridium botulinum produces seven different neurotoxins which are differentiated serologically by specific neutralization. The botulinum neurotoxins (BoNT) have been designated as serotypes A, B, C1, E, F, and G. Botulinum neurotoxins (BoNT) are the most toxic substances known and are the causative agents of the disease botulism. BoNT exert their action by inhibiting the release of the neurotransmitter acetylcholine at the neuromuscular junction (Habermann, E, et al., (1986), “Clostridial Neurotoxins: Handling and Action at the Cellular and Molecular Level,” Cur. Top. Microbiol. Immunol., 129:93–179; Schiavo, G., et al., (1992a), “Tetanus and Botulinum-B Neurotoxins Block Neurotransmitter Release by Proteolytic Cleavage of Synaptobrevin,” Nature, 359:832–835; Simpson, L. L., (1986), “Molecular Pharmacology of Botulinum Toxin and Tetanus Toxin,” Annu. Rev. Pharmacol. Taxicol., 26:427–453) which leads to a state of flaccid paralysis. Indeed, only a few molecules of toxin are required to abolish the action of a nerve cell. Polyclonal antibodies derived from a specific neurotoxin can neutralize the toxic effects of that toxin but will not cross-neutralize another toxin serotype. Thus, to protect against all seven toxins, one needs seven vaccines.
Human botulism poisoning is generally caused by type A, B, E or rarely, by type F toxin. Type A and B are highly poisonous proteins which resist digestion by the enzymes of the gastrointestinal tract. Foodborne botulism poisoning is caused by the toxins present in contaminated food, but wound and infant botulism are caused by in vivo growth in closed wounds and the gastrointestinal tract respectively. The toxins primarily act by inhibiting the neurotransmitter acetylcholine at the neuromuscular junction, causing paralysis. Another means for botulism poisoning to occur is the deliberate introduction of the toxin(s) into the environment as might occur in biological warfare or a terrorist attack. When the cause of botulism is produced by toxin rather than by in vivo infection the onset of neurologic symptoms is usually abrupt and occurs within 18 to 36 hours after ingestion. The most common immediate cause of death is respiratory failure due to diaphragmatic paralysis. Home canned foods are the most common sources of toxins. The most frequently implicated toxin is toxin A, which is responsible for more than 50% of morbidity resulting from botulinum toxin.
Botulinum and tetanus neurotoxins are a new class of zinc-endopeptidases that act selectively at discrete sites on three synaptosomal proteins of the neuroexocytotic apparatus. See Montecucco and Schiavo, 1995, and Schiavo, 1995, for review. These neurotoxins are the most potent of all the known toxins. The botulinum neurotoxins (BoNT), designed A–G, produced by seven immunologically distinct strains of Clostridium botulinum cause death by flaccid muscle paralysis at the neuromuscular junction. Extreme toxicity of these toxins and their lability in purified preparations have limited any detailed characterizations.
These neurotoxins are expressed as 150-kDa single polypeptides (termed dichains) containing a disulfide bond between the 50-kDa N-terminal light chain (LC) and the 100-kDa C-terminal heavy chain (HC). A post-translational cryptic cleavage generates the two chains connected by a disulfide bond. The LC contains the toxic, zinc-endopeptidase catalytic domain. The 100-kDa HC may be further proteolyzed into a 50-kDa N-terminal membrane-spanning domain (Hn) and a 50-kDa C-terminal receptor-binding domain (Hc).
With three functional domains, the mechanism of action of these neurotoxins is multiphasic: (1) The Hc domain plays a role in binding the toxins to specific receptors located exclusively on the peripheral cholinergic nerve endings (Black and Dolly, 1986). (2) The Hn domain is believed to participate in a receptor-mediated endocytotic pore formation in an acidic environment, allowing translocation of the catalytic LC into the cytosol. Reducing the disulfide bond connecting the LC with the H upon exposure to the cytosol or within the acidic endosome (Montal et al., 1992) releases the catalytic LC into the cytosol. (3) The LC then cleaves at specific sites of one of the three different soluable NSF attachment protein receptor (SNARE) proteins, synaptobrevin, syntaxin, or synaptosomal associated protein of 25 kDa (SNAP-25) (Blasi et al., 1993; Schiavo et al., 1993, 1994; Shone et al., 1993; Foran et al., 1996). These proteins are essential for synaptic vesicle fusion in exocytosis. Their proteolysis inhibits exocytosis and blocks acetylcholine secretion, leading ultimately to muscular paralysis. The LC itself is nontoxic because it cannot translocate through the cholinergic nerve ending into the cytosol. However, in digitonin-permeabilized chromaffin cells, the LC inhibits exocytosis (Bittner et al., 1989), and direct microinjection of the LC into the cytosol results in blockage of membrane exocytosis (Bittner et al., 1989; Bi et al., 1995).
The LC of all known clostridial neurotoxins contain the sequence HExxH that is characteristic of zinc-endoproteinases (Thompson et al., 1990). The essential role of zinc on the structure and catalysis of the neurotoxins is established (Fu et al., 1998). A unique feature of the neurotoxins' protease activity is their substrate requirement. Short peptides encompassing only the cleavage sites are not hydrolyzed (Foran et al., 1994; Shone and Roberts, 1994). A specific secondary and/or tertiary structure of the substrate is most probably recognized (Washbourne et al., 1997; Lebeda and Olson, 1994; Rossetto et al., 1994) rather than a primary structure alone, as is the case with most other proteases. Most importantly, their identified natural substrates are proteins involved in the fundamental process of exocytosis (Blasi et al., 1993; Schiavo et al., 1993, 1994; Shone et al., 1993; Foran et al., 1996). Light chain also is the target of an intensive effort to design drugs, inhibitors, and vaccines. A detailed understanding of its structure and function is thus very important.
The present invention describes the construction and overexpression of a synthetic gene for the nontoxic LC of BoNT/A in E. coli. The high level of expression obtained enabled purification of gram quantities of LC from 1 L of culture as well as extensive characterization. The preparation of the rBoNT/A LC was highly soluble, stable at 4° C. for at least 6 months, and had the expected enzymatic and functional properties. For the first time, a cysteine residue was tentatively identified in the vicinity of the active site which, when modified by mercuric compounds, led to complete loss of enzymatic activity.
The BoNTs and their LCs are targets of vaccine development, drug design, and mechanism studies because of their potential role in biological warfare, wide therapeutic applications, and potential to facilitate elucidation of the mechanism of membrane exocytosis. In spite of such immense importance, studies of the LC have been limited by its availability. Commercially available LC is prepared by separating it from the dichain toxins under denaturing conditions. These preparations therefore retain some contaminating toxicity of the dichain, have low solubility, and often begin to proteolytically degrade and start losing activity within hours of storage in solution.
The LC of serotype A has been separated and purified from the full-length toxin by QAE-Sephadex chromatography from 2 M urea; however, the preparation suffers from low solubility (Shone and Tranter, 1995). The LC of serotype C was similarly obtained at a level of <5 mg/10 L culture of C. botulinum (Syuto and Kubo, 1981). These preparations almost invariably contain contaminating full-length toxins, and the commercially available preparations precipitate from solution or undergo proteolytic degradation upon hours of storage in solution. More recently the LC of tetanus neurotoxin (Li et al., 1994) and of BoNT/A (Zhou et al., 1995) were expressed in E. coli as maltose-binding proteins and purified in 0.5 mg quantities from 1-L cultures (Zhou et al., 1995). However, the poor expression of the cloned products, probably due to rare codon usage in clostridial DNA (Makoff et al., 1989, Winkler and Wood, 1988), remained a major hurdle in obtaining adequate amount of the protein for structural and functional studies.
Most of the clostridial strains contain specific endogenous proteases which activate the toxins at a protease-sensitive loop located approximately one third of the way into the molecule from the amino-terminal end. Upon reduction and fractionation (electrophoretically or chromatographically), the two chains can be separated; one chain has a Mr of ˜100 kDa and is referred to as the heavy chain while the other has a Mr ˜50 kDa and is termed the light chain.
The mechanism of nerve intoxication is accomplished through the interplay of three key events, each of which is performed by a separate portion of the neurotoxin protein. First, the carboxy half of the heavy chain (fragment C or Hc is required for receptor-specific binding to cholinergic nerve cells (Black, J. D., et al., (1986), “Interaction of 125I-botulinum Neurotoxins with Nerve Terminals. I. Ultrastructural Autoradiographic Localization and Quantitation of Distinct Membrane Acceptors for Types A and B on Motor Nerves,” J. Cell Biol., 103:521–534; Nishiki, T.-I., et al., (1994), “Identification of Protein Receptor for Clostridium botulinum Type B Neurotoxin in Rat Brain Synaptosomes,” J. Biol. Chem., 269:10498–10503; Shone, C. C., et al., (1985), “Inactivation of Clostridium botulinum Type A Neurotoxin by Trypsin and Purification of Two Tryptic Fragments. Proteolytic Action Near the COOH-terminus of the Heavy Subunit Destroys Toxin-Binding Activity, Eur. J. Biochem., 151:75–82). Evidence suggests that polysialogangliosides (van Heyningen, W. E., (1968), “Tetanus,” Sci. Am., 218:69–77) could act as receptors for the toxins but the data supporting a specific receptor remains equivocal (Middlebrook, J. L., (1989), “Cell Surface Receptors for Protein Toxins,” Botulinum Neurotoxins and Tetanus Toxin, (Simpson, L. L., Ed.) pp. 95–119, Academic Press, New York). After binding, the toxin is internalized into an endosome through receptor-mediated endocyctosis (Shone, C. C., et al., (1987), “A 50-kDa Fragment from the NH2-terminus of the Heavy Subunit of Clostridium botulinum Type A Neurotoxin Forms Channels in Lipid Vesicles, Euro. J. Biochem., 167:175–180).
The amino terminal half of the heavy chain is believed to participate in the translocation mechanism of the light chain across the endosomal membrane (Simpson, 1986; Poulain, B., et al., (1991), “Heterologous Combinations of Heavy and Light Chains from Botulinum Neurotoxin A and Tetanus Toxin Inhibit Neurotransmitter Release in Aplysia,” J. Biol. Chem., 266:9580–9585; Montal, M. S., et al., (1992), “Identification of an Ion Channel-Forming Motif in the Primary Structure of Tetanus and Botulinum Neurotoxins,” FEBS, 313:12–18). The low pH environment of the endosome may trigger a conformational change in the translocation domain, thus forming a channel for the light chain.
The final event of intoxication involves enzymatic activity of the light chain, a zinc-dependent endoprotease (Schiavo, 1992a; Schiavo, G., et al., (1992b), “Tetanus Toxin is a Zinc Protein and its Inhibition of Neurotransmitter Release and Protease Activity Depend on Zinc,” EMBO J, 11:3577–3583), on key synaptic vesicle proteins (Schiavo, 1992a; Oguma, K., et al., (1995), “Structure and Function of Clostridium botulinum Toxins,” Microbiol. Immunol., 39:161–168; Schiavo, G., et al., (1993), “Identification of the Nerve Terminal Targets of Botulinum Neurotoxin Serotypes A, D, and E,” J. Biol. Chem., 268:23784–23787; Shone, C. C., et al., (1993), “Proteolytic Cleavage of Synthetic Fragments of Vesicle-Associated Membrane Protein, Isoform-2 by Botulinum Type B Neurotoxin,” Eur. J. Biochem., 217:965–971) necessary for neurotransmitter release. The light chains of BoNT serotypes A, C1, and E cleave SNAP-25 (synaptosomal-associated protein of M25,000), serotypes B, D, F, and G cleave vessicle-associated membrane protein (VAMP)/synaptobrevin (synaptic vesicle-associated membrane protein); and serotype C1 cleaves syntaxin. Inactivation of SNAP-25, VAMP, or syntaxin by BoNT leads to an inability of the nerve cells to release acetylcholine resulting in neuromuscular paralysis and possible death, if the condition remains untreated.
The majority of research related to botulinum toxin has focused on the development of vaccines. Currently, a pentavalent toxoid vaccine against serotypes A through E (Anderson, J. H., et al., (1981), “Clinical Evaluation of Botulinum Toxoids,” Biomedical Aspects of Botulism, (Lewis, G. E., Ed.), pp. 233–246, Academic Press, New York; Ellis, R. J., (1982), “Immunobiologic Agents and Drugs Available from the Centers for Disease Control. Descriptions, Recommendations, Adverse Reactions and Scrologic Response,” 3rd ed., Centers for Disease Control. Atlanta, Ga.; Fiock, M. A., et al., (1963), “Studies of Immunities to Toxins of Clostridium botulinum. IX. Immunologic Response of Man to Purified Pentavalent ABCDE Botulinum Toxoid,” J. Immunol., 90:697–702; Siegel, L. S., (1988), “Human Immune Response to Botulinum Pentavalent (ABCDE) Toxoid Determined by a Neutralization Test and by an Enzyme-Linked Immunosorbent Assay,” J. Clin. Microbiol., 26:2351–2356), available under Investigational New Drug (IND) status, is used to immunize specific populations of at-risk individuals, i.e., scientists and health care providers who handle BoNT and military personnel who may be subjected to weaponized forms of the toxin. Though serotypes A, B, and E are most associated with botulism outbreaks in humans, type F has also been diagnosed (Midura, T. F., et al., (1972), “Clostridium botulinum Type F: Isolation from Venison Jerky,” Appl. Microbiol., 24:165–167; Green, J., et al., (1983), “Human Botulism (Type F)—A Rare Type,” Am. J. Med., 75:893–895; Sonnabend, W. F., et al., (1987), “Intestinal Toxicoinfection by Clostridium botulinum Type F in an Adult. Case Associated with Guillian-Barre Syndrome,” Lancet, 1:357–361; Hatheway, C. L., (1976), “Toxoid of Clostridium botulinum Type F: Purification and Immunogenicity Studies,” Appl. Environ. Microbiol., 31:234–242). A separate monovalent toxoid vaccine against BoNTF is available under IND status. Hatheway demonstrated that the BoNTF toxoid could protect guinea pigs against a homologous challenge (Wadsworth, J. D. F., et al., (1990), “Botulinum Type F Neurotoxin,” Biochem. J., 268:123–128).
New-generation, recombinant vaccines have also been developed by USAMRIID (e.g. Dertzbaugh M T, Sep. 11, 2001, U.S. Pat. No. 6,287,566; U.S. Appln. Ser. No. 09/910,186 filed Jul. 20, 2001; and U.S. Application. Ser. No. 09/611,419 filed Jul. 6, 2000) and commercial sources (e.g. Ophidian Pharmaceuticals, Inc. Williams J A, Jul. 6, 1999, U.S. Pat. No. 5,919,665; using clones supplied by USAMRIID).
Most vaccine studies have focused on the botulinum toxin heavy chain, leaving the light chain largely ignored. In 1995, Zhou et al. discovered that a single mutation in the light chain of botulinum neurotoxin serotype A abolished its neurotoxicity and its ability to cleave SNAP-25, one of the natural substrates, when reconstituted with the heavy chain. See Zhou, L. et al., (1995), “Expression and Purification of Botulinum Neurotoxin A: A Single Mutation Abolishes its Cleavage of SNAP-25 and Neurotoxicity after Reconstitution with the Heavy Chain,” Biochem., 34:15175–15181.) This raised the possibility that the mutated light chain might have various research or therapeutic uses. Further research produced a recombinant light chain (Li, L. and Singh, B. R., (1999), “High-Level Expression, Purification, and Characterization of Recombinant Type A Botulinum Neurotoxin Light Chain,” Protein Expression and Purification, 17:339–344) and a construct comprising the minimum essential light chain domain (Kadkhodayan, S., et al., (2000), “Cloning, Expression, and One-Step Purification of the Minimal Essential Domain of the Light Chain of Botulinum Neurotoxin Type A,” Protein Expression and Purification, 19:125–130).
Recombinant production methods alleviate many of the problems associated with the toxoid, such as the need for a dedicated manufacturing facility. Presently, many cGMP facilities are in existence and available that could manufacture a recombinant product. There would be no need to culture large quantities of a hazardous toxin-producing bacterium. Production yields from a genetically engineered product are expected to be high. Recombinant products would be purer, less reactogenic, and more fully characterized. Thus, the cost of a recombinant product would be expected to be much lower than a toxoid because there would be no expenditures required to support a dedicated facility, and the higher production yields would reduce the cost of therapeutic and research products.
However, recombinant methods as described in the publications above do not yield optimal results because botulinum codons are not translated well in other organisms commonly used for production, such as E. coli or yeast. Furthermore, no easily translatable, recombinant form of the non-neurotoxic, mutated light chain presently exists. Recombinant forms of both functional and non-neurotoxic botulinum neurotoxin that may be translated efficiently in either E. coli or yeast are needed for research and therapeutic purposes.
Commercially available BoNT LC is prepared by separation from the di-chain toxins. These preparations, therefore, retain some contaminating toxicity, have low solubility, and undergo proteolytic degradation within hours and days of storage in solution. Many clinical disorders are presently being treated with a botulinum neurotoxin complex that is isolated from the bacterium, Clostridium botulinum. There is no data to demonstrate that the binding proteins play any role in the therapeutic effects of the drug. The binding proteins, however, probably contribute to the immunological response in those patients that become non-responsive to drug treatment. Recombinant products could be manufactured under conditions that are more amenable to product characterization. Chimeras of the drug product could also be produced by domain switching. Chimeras could potentially increase the number of potential useful drug products.
Recently, the BoNT LC of serotype A has been expressed as a maltose-binding protein and purified in 0.5 mg quantities from 1 liter culture (Zhou et a., 1995). The poor expression of the native gene was probably due to the high A+T composition found in the clostridial DNA.